FAIRFIELD — A former high school teacher in Fairfield has been charged with sexual assault after an investigation determined he had a sexual relationship with an underage female student, police said Thursday.

Glenn Mishuck, 47, of Bridgeport, was charged with 17 counts of second-degree sexual assault. Mishuck, who taught at Fairfield Ludlowe High School, was being held on $150,000 bond.

A message left with his attorney Thursday was not immediately returned.

Detectives received information Sept. 18 that there might be an inappropriate relationship between a teacher and student at the high school, police said. The investigation included numerous interviews and collection of electronic evidence, police said.

Police say Mishuck had a sexual relationship with a student under the age of 18 between July and September. Investigators said text messages between the teacher and student corroborated the allegations and that Mishuck engaged in at least 17 encounters with the student.

Mishuck came to police headquarters Thursday and was charged. He is due to be arraigned Nov. 12 in Bridgeport Superior Court.

School officials said Mishuck, an English teacher, was placed on paid administrative leave in September as soon as they learned of the allegation. They said he resigned Oct. 7.